1. Field of the Invention
The disclosure generally relates to a method of forming an element isolation film of a semiconductor device. More particularly, the disclosure relates to a method of forming an element isolation film of a semiconductor device in which an aluminum oxide film of a high wet etch rate is used as a pad oxide film, trenches are formed, and top and bottom edge portions of the trenches are then made rounded while removing some of the aluminum oxide film through a cleaning process.
2. Brief Description of Related Technology
In a manufacture process of semiconductor devices, an element isolation film is formed in order to isolate an active region and a field region or elements. As the devices become higher integrated and miniaturized, however, the element isolation film is formed by etching the semiconductor substrate at a given depth, thus forming trenches, and then burying the trenches with an insulating film. This element isolation film is formed in such a manner that a pad oxide film, a pad nitride film and a photoresist film are formed on the semiconductor substrate, the photoresist film is patterned using an isolation mask, the pad nitride film, the pad oxide film and the semiconductor substrate are etched at a given depth using the patterned photoresist film as a mask, thus forming trenches, and the trenches are buried with the insulating film. In this time; the pad nitride film is formed by LPCVD. If the pad nitride film is directly brought into contact with the semiconductor substrate, however, a crystal defect is caused in the semiconductor substrate because of stress between the pad nitride film and the semiconductor substrate. Thus, before the pad nitride film is formed, the pad oxide film is formed. In this time, the pad oxide film is formed using a thermal oxide film of a low wet etch rate.
Meanwhile, in a trench etch process for forming this trench type element isolation film, the roundness of the top edges of the trenches has an influence on characteristics such as a cell threshold voltage and refresh in a DRAM, and affects characteristics such as erase and standby current in a flash memory device. Accordingly, in order to make the top edges of the trench rounded, an etch process is performed using polymer generated when the pad nitride film is etched.
The etch process using polymer will be described in short. If the pad nitride film, the pad oxide film and the semiconductor substrate are etched so as to form the trench, an etch gas and the pad nitride film show a polymerization reaction. While polymer is generated, it is accumulated at the top edges of the trench. The accumulated polymer has an etch selective ratio different from that the semiconductor substrate. Thus, as the accumulated polymer serves as an etch stopper when the semiconductor substrate is etched, the top edges of the trench where polymer is accumulated are rarely etched compared to the center of the trench. Accordingly, the top edges of the trench are made rounded.
It is, however, very difficult to uniformly deposit polymer on the sidewalls of the pad oxide film so as to make the top of the trench rounded using polymer. Furthermore, there are problems in that foreign materials are generated in an etch apparatus as polymer is generated, and it is difficult to secure reappearance depending on a characteristic of an etch apparatus.
Meanwhile, the pad oxide film is formed using the thermal oxide film of a low wet etch rate. It is thus difficult to remove the pad oxide film in the cleaning process before the gate oxide film is formed. Furthermore, there is a problem in that a moat deepens because of a step between the field region and the active region because a gap filling material of a relatively high etch rate is first etched when the pad oxide film is removed.